Prothalamion is written in the conventional form of a marriage song. The poem begins with a description of the River Thames where Spenser finds two beautiful maidens. The poet proceeds to praise them and wishing them all the blessings for their marriages. The poem begins with a fine description of the day when on which he is writing the poem:

Calm was the day and through the trembling airThe sweet breathing Zephyrus did softly play.

The poet is standing near the Thames River and finds a group of nymphs with baskets collecting flowers for the new brides. The poet tells us that they are happily making the bridal crowns for Elizabeth and Katherine. He goes on his poem describing two swans at the Thames, relating it to the myth of Jove and Leda. According to the myth, Jove falls in love with Leda and comes to court her in the guise of a beautiful swan. The poet feels that the Thames has done justice to his nuptial song by "flowing softly" according to his request: "Sweet Thames run softly till I end my song". The poem is often grouped with Spenser's poem about his own marriage, the Epithalamion.